Add anonymizing of user applications feature

[wes-otf]

Would close #3441. Adds a new AnonymizedSubmission model that only contains minimal, non-PII datapoints. Also adds UI features for staff to select whether a user's application(s) should be deleted or converted to skeletoned/anonymized application.

The todos:

• Implement AnonymizedSubmission model & creation methods
• Handle manual individual submission deletions
• Handle manual bulk submission deletions/anonymization
• Handle submissions with individual user deletions
• Handle submissions with bulk user deletions
• Include anonymized submissions in the results dashboard
• Add unit tests for new functionality

Test Steps

• …

[wes-otf]

@frjo I'm going to add a unit test for the results view, some docs & do a final look over but it should be ready for review if you wanted to take a look before that

[wes-otf]

somewhat of a hacky solution to managing 2 models with one FilterView but seems to work?

[wes-otf]

seems like the # of SQL queries shoots up though so might need some optimizing

[frjo]

@wes-otf I rebased this with main to solve a small merge conflict.

[frjo]

@wes-otf When I was testing this I realised we have some existing issues with deleted/archive. When I started with fixing that more things followed as you can see in the PR now 😄.

• The backend is nearly untouched. This part seems to works really well!
• The wagtail admin part is nearly untouched as well, only added some classes for margins etc.

My changes:

1. Renamed everything skeleton -> anonymize, makes it easier to read the code I believe.
2. Added some tests.
3. Added "Anonymise" button in "MORE ACTIONS" section on submission detail. Anonymise also has its own confirm modal now.
4. Moved "Delete" button to "MORE ACTIONS" section for staff as well. I think this makes more sense.
5. Reverted changes to SubmissionDeleteView since they are not used any more.

[frjo]

The batch functions on submission/all only have a simple js alert while the same functions for a single submission has a nice modal. That is not logical. Will fix that in a separate PR.

[frjo]

This works very well in my testing, the backend seems solid. The anonymised submission has very little information left since all form_data is deleted. The feature talk about removing PII info, some might be surprised that almost nothing besides the value remain.

Maybe this can be communicated better?